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The sportier side of electric vehicles

The sportier side of electric vehicles

Technology News |
By eeNews Europe



Performance is a key requirement of electric motors and generators which are used in many applications, most importantly for electric vehicles, and is paramount in all sectors of motor sports: from KERS in Formula 1 to hybrid solutions in Le Mans cars. The main goals are to push performance and innovation to their limits, targets which have been publicly achieved on the race track in recent years. In precise terms, the aim is to achieve maximum performance at minimum weight, i.e. to optimise power density. Ultimately, the central technological requirements remain virtually identical, despite variations in the economic framework conditions. Innovative optimised drive technologies are the key pillars of eMobility.

The technological challenges of this new form of mobility are clearly demonstrated by the Formula Student championship, a competition in which teams of students from universities all over the world design and build single-seater racing cars and race them at iconic locations such as Silverstone and Hockenheim. However, the winner is not automatically the first to cross the finishing line in an individual race, but the team that accumulates the best overall scores for design and racing performance, based on both economic and environmental considerations.

Tough conditions

A range of dynamic disciplines are designed to push the racing cars to their limits. They must compete in a variety of categories including Acceleration (maximum acceleration and speed in drag racing from a standing start), Skid Pan (maximum lateral acceleration on a circular skid pan) and Autocross & Endurance (maximum traction and agility on the circuit and endurance racing).

From its origins as a race for vehicles with combustion engines, Formula Student was expanded in 2010 to include vehicles with pure electric drives. The resultant rise in interest serves as a testament to the dynamic pace of development in eMobility itself; after only two years, Formula Student Electric now has 85 entrants from 25 countries. While the design and construction of the vehicles and the participation in the individual races are the work of dedicated interdisciplinary university teams, sponsors contribute the necessary funding and materials. Since the Formula Student Electric was founded, the drive and control unit manufacturer AMK has supported several teams by providing synchronous motors and motor control systems.

A high power-to-weight ratio and lots of torque are the keys to success, delivering the necessary traction control for drag racing and maximising acceleration out of tight bends in circuit racing. Both the vehicle and its drive train must be designed for performance and lightness to combine straight line acceleration with agility.

Optimised power-to-weight ratio achieved by partnership and a new alloy

Since 2011, Vacuumschmelze has partnered AMK as a sponsor of Formula Student Electric with the aim of optimising electric motors with specific attention to their power-to-weight ratio. In line with the company’s slogan, "Advanced Materials – The Key to Progress", Vacuumschmelze produces rotor and stator assemblies from their CoFe alloys VACOFLUX® and VACODUR®. These materials exhibit significantly higher induction than classic electrical steel; for example, at 2.3 T their saturation magnetisation is 13% higher than that of electrical steel.

Table 1: Typical material properties of VACODUR 49 compared with electrical steel M270-50A.

 

This year, Vacuumschmelze completed the development of its new alloy, VACODUR 49. Specific heat treatments can be applied to this high-induction CoFe material, increasing its strength in order to handle the requirements of an electric motor or generator. For example, stator assemblies, which are exposed to lower mechanical stress, can be produced from an alloy with optimum magnetic properties but a relatively low yield strength of 210 MPa. However, higher-strength materials are frequently required for high-speed rotors and an appropriate heat treatment can increase the yield strength to up to 390 MPa, significantly higher than that of electrical steel (see chart 1). Independent of the strength of the material chosen, induction values dramatically outperform those of electrical steel, especially at lower field strengths. M270-50A electrical steel has an induction value of 1.49 T at a magnetic field strength of 2.5 kA/m, but at the same field strength, both variants of VACODUR 49 outperform electrical steel by about 50%, with values of 2.23 and 2.27 T respectively (see Fig. 1).

Fig. 1: Comparison of static initial magnetisation curves for VACODUR 49 and M270-50A electrical steel.

Induktion = Induction / magnetisch optimiert = magnetically optimised / mechanisch optimiert = mechanically optimised / Electroblech = Electrical steel / magnetische Feldstärke = Magnetic field strength

Motors with an extremely high power-to-weight ratio can therefore be built using Co-Fe alloys. Used by the aerospace industry for decades to save weight, these Co-Fe materials are now commonly adopted for applications in automation technology and high-end motor sports.

This is also the final link in the chain for the use of VACODUR 49 in the AMK motors for Formula Student Electric: starting with a Hightorque DT series liquid-cooled synchronous servo motor, the M270-50A electrical steel stator assemblies were replaced by assemblies made by Vacuumschmelze from VACODUR 49 (see Fig. 2).

Fig. 2: DT5-26-10-POW liquid-cooled synchronous motor by AMK with four stator assemblies of VACODUR 49.

The combination of AMK’s design concept with four stator assemblies made from VACODUR 49 produces a motor capable of impressive performance. Weighing in at a mere 8 kg, yet with a maximum output of 54 kW, it produces an increase in power of 32% over the standard electrical steel model (see Fig. 4). The motors conform to the specific conditions imposed by the competition rules with respect to the vehicle design and power output limits. In racing cars, maximum power output takes second place to torque, which is a key defining factor of the vehicle’s dynamic potential. With a maximum torque of 51 Nm, the motor delivered an improvement of 53% over its electrical steel counterpart. This high torque and low weight form the basis for the car’s optimum acceleration. Unlike combustion engines, electric motors can deliver torque from a standing start – the main reason for the clear superiority of electric-drive vehicles in drag races in recent years. From 10,000 rpm upwards, the motor electronics cap the torque and any further power output. However, this is not a hindrance in the race, the rules of which specify a maximum power output of 85 kW.

Fig. 4: Maximum torque and maximum power output of AMK’s DT5-26-10-POW synchronous motor using stator assembles of VACODUR 49 compared to M270-50A electrical steel.

The partnership of Vacuumschmelze and AMK is supporting three teams in this year’s Formula Student Electric: the University of Stuttgart (GreenTeam), Delft University of Technology (DUT Racing) and Leibniz University, Hanover (HorsePower). The Stuttgart and Hanover teams both have two motors on the rear axle, while Delft University of Technology has implemented a half-length version of the AMK concept into an all-wheel-drive design.

This year, team races have been held at Silverstone in the UK, Spielberg in Austria, Hockenheim in Germany and Barcelona in Spain. The peak performances and innovations made possible by the new alloy from Vacuumschmelze and AMK’s technical expertise are bound to drive one or other of the teams to the top. This is confirmed with the latest overall standings of two of the teams sponsored by Vacuumschmelze and AMK who have reached the top spots, with Delft University in 1st place and Stuttgart University in 3rd place. Zurich University, who are independently sponsored by Vacuumschmelze, achieved 2nd place in the standings.

The car:

Overview of the new E0711-3

Highlights: Total power output 102 KW (139 hp)

Weight: 229 kg

Acceleration to 100 km/h: 3 s

Batteries:

Rechargeable lithium-polymer

Number of cells: 432

Voltage: 600 V

Capacity: 6.9 kW/h

Drive train: 2 permanently excited water-cooled synchronous motors

Maximum power output: each 54 kWh, efficiency: 95%

Custom power electronics, torque vectoring

Mechanical details:

Hybrid monocoque (CFRP + tubular grid)

2-speed spur gears

13" rims with aluminium star

About the author:
Robert Brand was born in 1966 in Aschaffenburg/Germany. He is head of business development in the business unit "Materials & Parts" of VACUUMSCHMELZE GmbH & Co. KG. R. Brand has a PhD in physics and started his work history in research and development of soft magnetic materials.
Education as a physicist at the Technical University Darmstadt, Germany
Ph.D. degree in the field of disorder dynamics of plastic crystals at the University of Augsburg, Germany

 

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